Certain types of pumps employ a wet motor design in which some of the fluid being pumped is passed through a motor chamber to cool and lubricate the motor. Examples of such pumps are shown in U.S. Pat. Nos. 4,682,938 and 3,443,519. Another design is shown in U.S. Pat. No. 4,571,159. The pump shown in this patent includes a bladder which damps pressure and fluid flow variations.
Wet motor pumps are often used in aircraft applications. In many of these pumps a portion of the aircraft fuel is passed through the motor housing and either returned to the fuel tank or delivered to the engines. In many cases, multiple fuel tanks are used and a wet motor fuel pump is positioned in each tank. These fuel pumps run simultaneously while the aircraft is in operation.
The fuel level drops below the fuel inlet of the fuel pump when the fuel in the tank is used up. As fuel no longer passes through the motor cavity, the motor runs dry and the pump begins to heat up. The pump may be damaged unless fuel is restored or the pump is shut off.
To avoid damage due to overheating in the run dry condition, certain prior art pumps incorporate a thermal switch. The thermal switch shuts off electrical power to the pump when the temperature exceeds a set limit. The thermal switch restores electricity to run the pump only after the temperature drops below a safe limit. Thermal switches, however, have drawbacks in that they are expensive, bulky and add weight to the pump. They also limit the amount of electrical current that can be passed through the motor. Resetable thermal switches also present a possible area for pump failure.
Another type of aircraft pump employs a non-resetable thermal fuse which cuts off electricity to the pump once a set temperature is exceeded. While thermal fuses are smaller, more reliable and less expensive than resetable thermal switches, they must be replaced once they are tripped. This results in down time and labor expense.
Other approaches have been taken to extend the run dry capability of pumps. The pump shown in U.S. Pat. No. 3,960,467 provides for air flow through the pump housing once the fuel supply is used up. The flow of air helps to cool the internal components of the housing when the pump is in a run dry condition. It is doubtful that such a system could be successful when the pump is enclosed within a fuel tank.
Thus, there exists the need for a fuel pump that can run dry for extended periods of time without reaching a temperature that may cause damage to the pump.